MIMO is abbreviated from Multiple-Input Multiple-Output, and means a method for improving efficiency in data transmission and reception by using multiple transmitting antennas and multiple receiving antennas instead of a single transmitting antenna and a single receiving antenna. Namely, the MIMO technology is to increase capacity or improve throughput by using multiple antennas in a transmitter or receiver of a wireless communication system. Herein, the MIMO technology may be referred to as multi-antenna technology.
Examples of a MIMO scheme include a Single CodeWord (SCW) scheme and a Multiple CodeWord (MCW) scheme, wherein the SCW scheme is intended to transmit N number of data streams at the same time by using a single channel encoding block, and the MCW scheme is intended to transmit N number of data streams by using M (M is always smaller than or equal to N) number of channel encoding blocks. In this case, each channel encoding block generates independent codewords, each of which is designed to enable independent error detection.
In a system that transmits multiple codewords, a receiver is required to notify a transmitter of success/failure in detection (or decoding) of each codeword. To this end, the receiver may transmit a hybrid automatic repeat request (HARQ) acknowledgement/non-acknowledgement (ACK/NACK) signal for each codeword to the transmitter.
In case of uplink data transmission through the existing single antenna, single codeword transmission may be supported. The receiver (for example, base station) that receives a single codeword from the transmitter (for example, user equipment) may transmit the HARQ ACK/NACK signal to the transmitter through a physical hybrid automatic repeat request indicator channel (HARQ PHICH).